1. Field of the Invention
The present invention relates to an on-vehicle automatic loudness control apparatus used in on-vehicle audio equipment.
2. Prior Art
In on-vehicle audio equipment, the reproduced audio signal is often faint in the background of engine noise and so-called road noise developed by the tires running on road surfaces. There have been available audio equipment adapted to automatically control the loudness of the output in accordance with the environmental noise in a passenger space.
FIG. 2 shows one such type of on-vehicle automatic loudness control apparatus which includes a signal source H such as a tuner or a tape player and an adapter A for automatically controlling the loudness. The head unit H includes primarily an audio signal source 1 of an audio signal reproduced through the tape player or the tuner and a first volume control circuit 2 for controlling the audio output level from the signal source 1. A microphone 3 is disposed at a suitable location in the car room to detect the noise. A lowpass filter 4 amplifies the output from the microphone 3 and extracts only the low frequency components. The output of the filter 4 is supplied to a noise detector 5 which provides a dc signal indicative of the level of the noise. An audio signal detector 7 receives the output of the volume control circuit 2 and provides a dc signal indicative of the level of the audio signal. A relative level detection circuit (subtracter) 8 compares the level of the output of detector 7 with that of the noise detector 5 to output a relative output, which in turn is converted into a digital signal by an A/D converter 9. Then, the output of the A/D converter 9 is fed to a microprocessor 10 which provides a volume-controlling signal. A second volume control circuit 12 receives the output from the first volume control circuit 2 and controls the output signal level from the volume control circuit 2 in accordance with the volume-controlling signal.
With the prior art automatic loudness control apparatus of the above-described construction, the volume control circuit 2 is controlled by the user to set the input level to the volume control circuit 12. This set level determines a desired speaker output level. The volume control circuit 12 is controlled by the microprocessor 10 in accordance with the output of the relative level detection circuit 8 such that the output of the circuit 12 is adjusted back and forth through an output level corresponding to the input level thereto. Thus, the output from the speaker is adjusted back and forth through the desired speaker output level in accordance with the environmental noise level.
It should be noted that the audio signal goes through the two volume control circuits connected in series, usually of an electronic type. These circuits must be of a low noise type and also must not produce so-called pop noise when operated to control audio signal level therethrough. Such circuits are necessarily expensive.